In a continuously operating extruder, the concrete mix is extruded with the help of auger feeders into a mold or through nozzles, whereby the casting machine is propelled along the casting bed by the reaction forces imposed on the auger feeders. The ready-cast product remains on the casting bed. Conventionally, the casting operation over entire length of the long casting bed is arranged to occur according to the needs of that one of the individual pieces having the most stringent specifications in the entire casting run. Then, this specific slab dictates the pretensioning steel tendons, their pretension forces and the concrete mix grade to be used in the product. Resultingly, an extra high quantity of reinforcing steel and concrete mix of an unnecessarily high grade are wasted in many of the other slabs to be cut from the long slab cast on the bed. Such extra costs can be reduced, e.g., by way of using a concrete mix grade individually adjusted to meet the specifications of each product.
A single casting bed may also be used for casting concrete products of different quality grades by continuous extrusion if the casting machine is equipped with a compartmentalized feeder hopper, wherein the discharge of concrete mix from the compartments of the feeder hopper takes place controlled by casting length covered on the casting bed.
A two-compartment slip-form casting technique well known in the art is the so-called slideformer method, wherein two or three layers of concrete are cast atop one another in order to obtain a desired end product. All the concrete layers are fed from different feed hoppers and, generally, all the hoppers are filled with the same concrete mix grade. In exceptional cases, a different concrete grade such as suitable for making exposed aggregate products, for instance, is cast into either the bottom layer of the slab or the surface layer thereof. In the prior-art method, each one of the cast concrete mix layers is compacted separately.
In the method and apparatus described in FI Patent Application No. 991165 for manufacture of fiber-reinforced concrete products by extruder casting technique, a slip-form casting machine is disclosed having its feed hopper partitioned into at least two compartments and equipped with a control gate adapted to cover alternatingly as required the bottom discharge opening of either one of the two feed compartments. In this system, one compartment of the feed hopper contains standard-grade concrete mix, while the other compartment is filled with fiber-reinforced mix. In this fashion, the control gate of the feed hopper discharge openings can be controlled to adjust the feed ratio of the different concrete mixes so as to obtain a desired kind of end product. The primary function of this apparatus is to provide nonhomogeneous distribution of fiber reinforcement in the cross section of the end product.